Block copolymers of the A-B-A type, wherein A is typically a monovinyl aromatic hydrocarbon polymer block and B is a conjugated diolefin polymer block, are well known thermoplastic elastomers.
These block copolymers need not be chemically crosslinked to exhibit elasticity of the same degree as that of vulcanized rubber, and like conventional thermoplastics, the block copolymers become plastic on application of heat and can be easily shaped or otherwise processed. Because of these advantages, the aforementioned block copolymers are used to make various shaped articles which are conventionally formed from vulcanized rubber, such as footwears, sheets, belts, tires, and toys. Other use of these block copolymers are as raw materials for making adhesives or as modifiers for incorporation in plastics.
As a representative method for producing thermoplastic elastomers made of these block copolymers, there is a known method in which a monovinyl aromatic hydrocarbon is polymerized in a polymerization medium in the presence of an organomonolithium compound as a polymerization initiator; a conjugated diolefin is then added to perform block copolymerization; and a monovinyl aromatic hydrocarbon is further added to perform polymerization, or in the alternative, a dialkenyl aromatic hydrocarbon or a dihalogenated hydrocarbon is added to perform coupling, to thereby obtain a block copolymer of the A-B-A' type, wherein A is a monovinyl aromatic hydrocarbon polymer block, B is a conjugated diolefin polymer block, and A' is either the same as A or a polymer block derived from the dialkenyl aromatic hydrocarbon or dihalogenated hydrocarbon, as described, for example, in Japanese Patent Publication Nos. 23798/1965, 24914/1965, and 24915/1965.
In order to polymerize the monovinyl aromatic hydrocarbon in the first step, the aforementioned known method employs as the polymerization medium a solvent capable of dissolving the resulting monovinyl aromatic hydrocarbon polymer, such as aromatic hydrocarbon solvents (e.g., benzene and toluene), alicyclic hydrocarbon solvents (e.g., cyclohexane and methylcyclohexane), and ether solvents (e.g., tetrahydrofuran). These solvents, however, have high toxicity or boil at high temperatures and present various problems when they are used on an industrial scale. In addition, the either solvents will increase the proportion of the 1,2-bond or 3,4-bond on the block B of the resulting block copolymer, with the result that the product tends to reduce elasticity, especially elastic recovery at low temperatures.
Japanese Patent Publication No. 19286/1961 describes a method in which a conjugated diolefin is polymerized using an organodilithium compound (e.g., dilithiostilbene) as a polymerization initiator, and the resulting active diolefin polymer is copolymerized with a monovinyl aromatic hydrocarbon. Further, Japanese Patent Publication No. 3990/1971 describes a method in which polymerization of a conjugated diolefin and a monovinyl aromatic hydrocarbon is carried out using as a polymerization initiator an organomonolithium compound solution of a chained aliphatic hydrocarbon in which a block or graft copolymer composed of a conjugated diolefin polymer block and a monovinyl aromatic hydrocarbon polymer block or a mixture of these block and graft copolymers is dissolved or stably dispersed.
The former method is disadvantageous in that it is difficult to obtain the organodilithium compound on an industrial scale and that if the organodilithium compound whose purity is once reduced as a result of, for example, its recycled use is employed, the resulting thermoplastic elastomer of the A-B-A' type has insufficient physical properties, especially small mechanical strength. In addition, the synthesis of the organodilithium compound usually involves the use of an ether solvent, and this ether solvent, entering into the polymerization system, may increase the proportion of the 1,2- or 3,4-bond on the block B of the block copolymer, to thereby reduce the elasticity of the block copolymer, especially its elastic recovery at low temperatures.
The latter method requires preliminary production of the intended block or graft copolymer or mixture of block and graft copolymers, and therefore, this results in prolonged process time and reduced productivity.
As described above, the known methods for the production of the block copolymer of the A-B-A type suffer from one or more defects or limitations in their industrial operation, and because of inevitable increase in production costs or need for making special provisions to ensure hygenic conditions for the workers, a final product either has degraded quality or lacks good reproducibility of quality. These present a substantial bar against commercial use of the thermoplastic elastomer of the A-B-A type.